1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to the apparatus wherein at least a photoconductor drum and a fuser may be operative independently of each other.
2. Description of the Prior Art
Hitherto, in an image forming apparatus such as an electrophotographic printer, an electrically charged photoconductor drum is illuminated with a light source to form an electrostatic latent image on a surface thereof, a developing is performed upon adhesion of a toner to the electrostatic latent image, and then the thus obtained toner image is transferred to a recording material.
FIG. 9 shows a conceptual view for explanation of an electrophotographic process in the image forming apparatus. The electrophotographic process comprises a photoconductor drum 11 having a photoconductive surface on an outer periphery thereof, an electrostatic charger 12 for uniformly charging the photoconductor drum 11, a light emitting diode array or exposure 13 for forming an electrostatic latent image corresponding to image data on the photoconductive surface, a developing unit 14 for providing electrostatic adhesion of a toner to the electrostatic latent image, a transfer unit 15 for transferring a toner image to a sheet of paper, a fuser 16 for heating and fusing the transferred toner image to penetrate through fibers of the paper, and a cleaner 17 for removing residual electrostatic charge and toner on the photoconductive surface.
During one rotation of the photoconductor drum 11, the respective processes of charging, exposing, developing, transferring, discharging and cleaning are carried out in the order named, and the fixing process is performed on a printing paper.
The developing unit 14 varies in its structure depending on a toner to be used, which consists of a single component or two components. In case of use of a toner consisting of a single component, the developing unit 14 comprises a toner replenishment roller for replenishing the toner, a developing roller for providing adhesion of the toner to the electrostatic latent image on the photoconductor drum 11 to form a toner visible image and a blade for providing a unity of thickness of the toner adhered to the developing roller.
The toner replenishment roller is rotated in conjunction with the developing roller in a slidable relation. Thus, there is generated frictional electrostatic charge between the developing roller and the toner, so that a surface of the developing roller is charged with the positive electricity (+) and the toner is charged with the negative electricity (-). As a result, the toner adheres to the developing roller. The toner adhered to the developing roller is deleted by the blade so that a thin film of the toner is formed on the surface of the developing roller.
Thus, the toner adheres to an exposure portion of the photoconductor drum 11 at a contact portion of the photoconductor drum 11 and the developing roller, so that the electrostatic latent image is visualized. As stated above, charging of the toner is performed by the frictional electrostatic charge. This brings uneven density in polarity of charge. As a result, there would occur so-called "background" wherein a toner adheres to a portion which is to be the original white part, and a scatter of the toner, thereby inviting deterioration of a quality of an image and waste of the toner.
After completion of the transfer process, residual toner, which has remained on the photoconductive surface of the photoconductor drum 11 without being transferred, is removed by a cleaning brush or a cleaning blade of the cleaner 17, so that the photoconductive surface is in cleaned. Since the cleaner 17 is contact with the photoconductive surface, it has a strong effect on the span of life of the photoconductor drum 11.
Generally, the fuser 16 employs a pressure roll fixing scheme, and comprises a roller self containing a heater subjected to a Teflon coating process, a pressure roller consisting of a silicone rubber, a guide for preventing a paper after fixing from being rolled by the heater roller, and so on.
The span of life of the fuser 16 is shortened by an injury to the Teflon coating owing to such an involvement that the fused toner penetrates between the heater roller and the guide, or exhaustion of the Teflon coating and the silicone rubber owing to the friction at a paper running.
Thus, the amount of consumed toner, the span of life of the photoconductor drum 11 and the span of life of the fuser 16 are determined in accordance with the number of rotations in each of which a series of processes are carried out. Consequently, it is necessary to reduce the useless operations as much as possible.
FIG. 10 shows a schematic representation of the conventional image forming apparatus. In the figure, a printing apparatus 1 includes in control unit 2 an external connection interface 3 constructed with a Centronics Interface and so on. Printing data from a terminal unit 9 are entered through the external connection interface 3.
The control unit 2 is provided with, in addition to the external connection interface 3, a receiving buffer 4, a microprocessor 5, a font memory 6, an image memory 7 and a printer interface 8.
The control unit 2 is connected through the printer interface 8 to a printer unit 10. The printer unit 10 is provided with, as stated above, the photoconductor drum 11 having a photoconductive surface on an outer periphery thereof, the electrostatic charger 12 for uniformly charging the photoconductor drum 11, the light emitting diode array (exposure) 13 for forming an electrostatic latent image corresponding to image data on the photoconductive surface, the developing unit 14 for providing electrostatic adhesion of a toner to the electrostatic latent image, the transfer unit 15 for transferring a toner image to a printing paper, the fuser 16 for heating and fusing the transferred toner image to penetrate through fibers of the printing paper, and a cleaner 17 for removing residual electrostatic charge and toner on the photoconductive surface.
Further, the printer unit 10 is provided with a printing paper feeding unit 18 for automatically feeding a fixed type of printing paper, a hopping roller 19 for supplying the printing paper from the printing paper feeding unit 18, a hopping roller clutch 20, a paper feed sensor 22 for sensing the printing paper supplied from the printing paper feeding unit 18, a registering roller 23 for transporting the printing paper, a delivery roller 25 for delivering the printed printing paper to a delivery unit 27, a paper output sensor 24 for sensing delivery of the printing paper, a main drive motor 21, which may be a stepping motor, for paper feed, paper transfer, driving of the photoconductor drum 11 and so on, power transmission means 30 such as a transmission gear and a transmission belt for power transmission, and printing mechanism control 26 for controlling the whole mechanism of printer unit 10 and performing communication with the control unit 2.
The control unit 2 and printer unit 10 are connected through a serial interface 28 for performing communication therebetween and a video interface 29 for transferring and controlling printing data formed with dot image data.
In the printing apparatus 1, when the printing data is entered from a host computer, not shown, through the terminal unit 9 and the external connection interface unit 3, the data is taken in the receiving buffer 4.
Next, microprocessor 5 reads character codes included in the printing data, and generates dot image data for printing referring to the font memory 6.
The dot image data for printing is edited, for example, as dot image data for printing corresponding to a page of printing paper, and then written into the image memory 7.
In case of transmission of the image data from the host computer, not shown, a dot image developing processing is performed at the host computer side. Thus, not only does it take time until a transmission starts, but the data transmission takes time, since the image data is of a large capacity. However, in this case, it is sufficient for the microprocessor 5 to write the received image data into a predetermined address of the image memory 7 as it is.
On the other hand, in case of a business graph printing and so on, data are transmits from the host computer in form of a graphic command. Thus, the microprocessor 5 performs an editing operation for the data and then writes the edited data into a predetermined address of the image memory 7. In this case, the microprocessor 5 performs a bit map developing processing, and thus it takes much time to write a page of dot image data into the image memory 7.
Thus, when the dot image data for printing has been completed, a printing paper is transferred from the printing paper feeding unit 18 toward the the transfer unit 15. During transfer of the printing paper, the dot image data for printing drives the light emitting diode array 13 to form the latent image on the outer periphery of the photoconductor drum 11. When the photoconductor drum 11 rotated in the direction of arrow 111, the latent image is developed by the developing unit 14 and the developed image is transferred to the printing paper in the transfer unit 15. The printing paper is fixed by the fuser 16 and then discharged toward the carrying out unit 27. Thus, printed on the printing paper is information based on printing data received from the host computer.
Next, the conventional image forming apparatus will be explained more in detail, referring to FIGS. 11-14, hereinafter.
FIG. 11 shows an operational flow of a control unit of the conventional image forming apparatus. FIG. 12 is an operational flow chart of a printing unit of the conventional image forming apparatus. FIG. 13 is a time chart of the conventional image forming apparatus, and FIG. 14 is a diagram used for the explanation of the printing unit of the conventional image forming apparatus.
Now referring to FIG. 11, in steps S1 and S2, first, upon receipt of printing data transmitted from the host computer, the control unit 2 performs a page of receiving processing. In steps S3 and S4, an editing processing is performed to form a page of dot image data. In steps S6-S7, a paper feed ready signal, which is transmitted from the printer unit 10 through the serial interface 28, is discriminated, and if possible, a print start command is transmitted through the serial interface 28 in a timing shown in FIG. 13(c) to start a real printing processing.
Next, referring to FIG. 12A, in steps S10-S12, the printing mechanism control 26 determines, upon receipt of the print start command, as to whether the printer unit 10 is in condition of a rest, and if in the rest condition, there starts a printing preliminary operation for performing a real printing operation. In the printing preliminary operation. the main drive motor 21 turns on, and the photoconductor drum 11, the registering roller 23, the fuser 16 and the delivery roller 25 are driven by coupling means such as power transmission means 30, for example, a transmission gear and belt, for power transmission (refer to FIG. 14A). Thus, there are provided the preliminary operations such as a primary charge on the photoconductor drum 11, a toner charge by toner agitating in the developing unit 14, and providing a constant fusing temperature. Such a preliminary operation is carried out during (time TR) one to two revolutions of the photoconductor drum 11.
In step S13, the printing mechanism control 26 turns on the hopping roller clutch 20 and connects the power of the main drive motor 21 to the hopping roller 19 so as to start taking out of the printing paper 31 from the printing paper feeding unit 18 (refer to FIG. 14B).
In step S14, the printing paper 31 taken out from the printing paper feeding unit 18 runs on a paper carrying path, the paper feed sensor 22 informs the printing mechanism control 26 of a paper feed sensor turn-on signal at the time (time TH) when the paper feed sensor 22 detects the paper presence (refer to FIG. 14C).
In steps S15-S16, when the printing mechanism control 26 recognizes the fact that the printing paper 31 is detected by the paper feed sensor 22 and be transferred by the registering roller 23, the hopping roller clutch 20 is turned on to provide conditions for allowance of a real printing.
In step S17, at the time point (time TW) when a latent image is formed by the light emitting diode array 13, a toner image is formed by the developing unit 14, and there is provided such a relation that a distance that a distance l1 until the portion of the surface of photoconductor drum 11, on which the toner image is formed, reaches the transfer unit 15 for transferring the toner image to the printing paper, equals a remaining carrying distance l1 for the printing paper until it reaches the transfer unit 15. The printing mechanism control 26 starts a real printing processing to request the control unit 2 to send the dot image data for printing through the video interface 29 (refer to FIG. 14D). When the real printing processing starts, the dot image data for printing is transmitted from the control unit through the video interface 29 and the light emitting diode array 13 is driven, so that the latent image is formed on the outer periphery of the photoconductor drum 11. The photoconductor drum 11 rotates in direction of arrow 111 in synchronism with the paper transfer, the latent image is developed by the developing unit 14 to form the toner image, and the toner image is transferred to the printing paper 31 in the transfer unit 15 (refer to FIG. 14E).
In steps S18-S20, while a series of operations from the latent image formation to the transfer are sequentially performed, the printing mechanism control 26 monitors a running condition of the printing paper 31 and informs the control unit 2 of a next page feed ready signal through the serial interface 28, upon detection of turn on of the paper output sensor 24 and turn off of the paper feed sensor 22 (refer to FIG. 14F).
In steps S21-S22, when the printing mechanism control 26 detects turn on of the paper output sensor 24, the control unit 2 transmits a printing start command for a next page to the printing mechanism control 26 through the serial interface 28, if an editing processing for the next page printing data has been completed.
In steps S23-S24, the printing mechanism control 26 starts a next page paper feed processing, upon receipt of the printing start command for the next page, and transmits a paper feed ready signal off to the control unit 2.
In a case where no printing data for a next page exists in the control unit 2, or in a case where the editing processing has not completed in the control unit 2, there is provided a discharge waiting condition for the printing paper in course of the real printing processing. When the transfer of the processed page has been completed (refer to FIG. 14G), and the printing paper is moved for distance 12 from the transfer unit 15, in other word, the photoconductor drum 11 moves for distance 12 from the transfer unit 15 to the cleaner 17, residual electrostatic charge and toner on a surface of the photoconductor drum 11 are removed by the cleaner 17, and the cleaning processing for the page is terminated (refer to FIG. 14H). The printing paper 31 after transfer is fixed by the fuser 16 (refer to FIG. 14I). When the page has passed through the paper output sensor 24 (refer to FIG. 14I), the printing mechanism control 26 detects turn off of the paper output sensor 24 in step S21 and turns off the main drive motor 21 in step S25 so as to terminate the real printing processing. During a period of time until the main drive motor 21 is turned off, the photoconductor drum 11, the developing unit 14, the registering roller 23, the fuser 16 and the delivery roller 25, which are coupled with the main drive motor 21 through the belt, gear and so on, continue to be driven.
In the image forming apparatus arranged as stated above, however, the printing paper feeding unit 18, the photoconductor drum 11, the developing unit 14, the registering roller 23, the delivery roller 25 and so on are driven by a single power source, and thus those elements are simultaneously driving-started and driving-stopped. That is, according to the conventional image forming apparatus as stated above, it would be impossible to perform, only at the necessary time, the charging for the photoconductor drum 11, the charging for the toner within the developing unit 14, the developing operation, the cleaning operation, and the fixing operation.
It is necessary for the photoconductor drum 11 and the developing unit 14 to provide time TR needed for a preliminary operation (performed only when one page printing), printing paper transfer time TW until start of print data writing, printing paper transfer time T11 from a point of time of writing of the printing data until the transfer, and printing paper passage time TP, but unnecessary to provide the other times such as printing paper feeding time TH and discharge time (given by subtraction of cleaning time from TS). Reversely, it is unnecessary for the fuser 16 to provide those times TR, TH, TW and T11. Particularly, in case of one page printing/one job, or in a case where receiving processing and editing processing times are long, needless operation would be performed.
Further, in a case where there is provided a long distance for the printing paper transfer from the printing paper feeding unit 18 to the photoconductor drum 11 as an image forming section, or in a case where there is provided a long distance for the printing paper transfer from the photoconductor drum 11 or the fuser 16 to the discharge section, and so on, life of the photoconductor drum 11 and the fuser 16 would be shortened, and further increases the amount of consumed toner.